1. Technical Field
The present invention relates to a lead-free electrical solder composition having a primary metal powder and an additive metal powder mixed into a solder paste. The metal powder may be either an elemental metal or a metal alloy. More specifically, the present invention relates to a method of manufacturing a lead-free electrical solder having a primary metal powder and a higher melting temperature, additive metal powder mixed therein.
2. Description of the Related Art
It is known to manufacture an electrical solder paste that is made of metal or alloy particles mixed with flux. In such conventional solder paste, the metal particles are made by melting a metal or alloy ingot and atomizing the molten metal into fine particles of suitable size distribution. Such solder paste can be used for electronics packaging. The solder paste is first printed onto the bond pad of the substrate whereupon the electrical circuit resides. The substrate material may be, for example, fiber-reinforced epoxy resin (for example FR4) or ceramics (for example alumina). A solder paste is applied to the substrate and electronic components are placed onto the printed solder. The assembly is sent through an oven with a prescribed temperature profile, such that the metal particles in the solder paste will melt, flow and spread to the component termination surface and then re-solidify as the solder cools. The solder forms joints between the electronic components and the electronic circuit on the substrate. The solder joints serve as electrical interconnects between the electronic components and the electrical circuitry on the substrate of the electronic module.
The electronic module both heats and cools from the current passing through the electronic component and from the exposure to ambient temperatures. Because the electronic component, solder, and substrate are each made of different materials, they expand and contract differently in response to such temperature variations. As a result, large thermomechanical loading including stress and strain is imposed on the solder. The situation is especially severe in automotive applications because the temperature variations are very large and are repeated thousands of times over the life of the vehicle. Coupled with the vibrations from the road, the interconnects are exposed to very large stress and strain. Under such conditions, the solder microstructure coarsens, and weakens the solder joint. Eventually, the solder interconnect may develop fatigue failures by crack initiation and propagation through the solder joint, rendering the electronic module non-functional. It is therefore highly desirable to produce solder joints that are resistant to such fatigue failures.
Tin-lead solders (63%Sn-37%Pb, or 62%Sn-36%Pb-2%Ag) are widely used in current production of automotive electronic modules. More recently, tin-silver solder (96.5%Sn-3.5%Ag) has been used as a lead-free solder for automotive electronics. Solder manufacturing processes such as alloy making, atomization and flux formulation as well as soldering process parameters such as printing and reflow have been well established. However, these solders frequently suffer from the fatigue failures as described above in automotive applications, especially for large size electronic components.
In order to enhance the reliability of the solder joint, two approaches can be taken. In the first approach (the xe2x80x98material approachxe2x80x99), a different and more fatigue resistant solder material can be used. Such solder materials should be stable and resistant to coarsening and fatigue damage. The choice of materials for this approach, however, is limited by the fact that the solder must also possess chemical and physical characteristics (such as melting temperature, wettability, etc.) suitable for soldering and properties (such as corrosion resistance, thermal/electrical conductivities, etc.) required for the intended application. It is therefore advantageous for any new solders to be based on the currently used Snxe2x80x94Pb and Snxe2x80x94Ag solders. The second approach (the xe2x80x98geometry approachxe2x80x99) is to produce solder joints that have a large stand-off height (the height of the solder between the substrate and the component). In the case of leadless chip components, for example, the fatigue life of the solder joint has been shown roughly to be inversely proportional to the maximum shear strain in the stand-off region of the solder joint, which arises as a result of temperature variations due to the difference in the coefficient of thermal expansion (CTE) for the various materials involved; this maximum shear strain is in turn, generally speaking, inversely proportional to the stand-off height. Therefore, increasing the stand-off height is expected to increase the solder joint fatigue life. This however is not simple to achieve, because on the one hand, the amount of solder paste that can be deposited on the bond pad is limited by the printing process in high volume production due to the presence of fine pitch devices on the same module; and on the other hand, when the solder paste melts, it flows and spreads to the end of the component termination therefore reducing the amount of solder that is left between the component and the substrate.
U.S. Pat. No. 5,127,969 (""969) teaches an electrical solder ingot containing a metal alloy and a reinforcement material mixed therein, and a method for its manufacture. The reinforcement material may be either a metal, metal alloy or carbon fiber. The reinforcement material is formed into a particulate or fibrous form and mixed with the solder alloy. The electrical solder is heated to a molten or semi-solid state and reinforcement material is mixed therein under xe2x80x9cvigorous shearingxe2x80x9d. However, it is believed that the composition taught in the ""969 patent cannot be used for the application intended for the present invention. In any soldering application, the solder ingot will have to be remelted. For example, in solder paste manufacturing, the solder ingot has to be remelted and atomized into fine particles in the range of 30-50 microns. This process however will completely separate the solder alloy and the reinforcement material. Notwithstanding the assertion that the composite retains its desired microstructure after remelting, it is believed that remelting the composite solder ingot will cause the distributed reinforcement particles to completely separate from the solder alloy. The only way of using the ""969 patent may be in the form of solder wires in hand soldering; but even in hand soldering, separation is still expected to occur. Further, hand soldering is a low volume, low speed operation and is therefore not the object of the present invention.
U.S. Pat. No. 5,350,105 (""105) teaches a solder connection having a first solder portion with a first melting point and a second solder portion with a second melting point that is higher than the first melting point. The purpose of this construction is for the second solder portion to indicate when the first solder portion has flowed and/or to control the flow of the first solder portion. The ""105 patent further teaches the use of two solder alloys that remain separate. These alloys do not improve the mechanical, physical or electrical properties of the resulting solder connection.
The present invention seeks to simultaneously utilize the two approaches described previously (the materials approach and the geometry approach) to enhance solder interconnect fatigue life. It is therefore desirable to have a composite solder that retains the general physical properties of the currently used solder but also have improved mechanical performance over the life of the electronic module. It is further desirable that the composite solder be compatible with existing high volume solder manufacturing methods. Further, it is desired that the solder be applied as a paste using high speed printing and transfer equipment, for reflow soldering which is the predominant electronic packaging process preferred for automotive applications. It is further desirable that the solder composition be free of lead.
Currently, most solder alloys are produced by mixing a predetermined proportion of ingredients in a metal or ceramic crucible and heating the mixture until the metals become molten. The molten metal is then poured into molds, and ingots are made when the metal cools and solidifies. To make solder paste, the ingots are re-melted and atomized in a controlled atmosphere to make a fine solder powder. The resulting powder is mixed with flux to form a solder paste. The solder paste is applied to a circuit board through stencil or screen printing, and electronic components are placed on the printed paste. The circuit board is then placed into an oven and heated to a temperature sufficient to cause the solder powder to melt and flow. When the temperature decreases, the molten solder solidifies forming solder joints with the electronic components.
The present invention relates to a method of manufacturing a lead-free electrical solder paste that includes, in addition to the primary solder powder used in conventional solder paste, an additive metal powder component that does not melt (except for a certain amount of dissolution) during the soldering process. This additive metal or alloy powder component causes the solder to be more viscous and therefore creates solder joints with large stand-off heights. Further, the solder joints are significantly more resistant to fatigue damage over the life of the electronic module.
The lead-free electrical solder is manufactured by mixing a first metal powder (the primary powder) and a second metal powder (the additive powder). Metal powders may be either elemental metal or a metal alloy. The powders are made by conventional methods such as re-melting the metal ingot followed by atomization. The primary powder is the same as is used in conventional solder paste. The additive powder has a melting point substantially higher than the melting point of the primary powder. The primary powder and the additive powder are mixed in predetermined proportions with flux to form a solder paste. The method of mixing powders with flux to form a solder paste is well established. Conventional fluxes are used, which may include ingredients that serve as vehicles, oxide-removal agents, etc. Solder paste characteristics (e.g. thixotropy) suitable for storage, printing, and other process requirements are well established.
As a result of developing the method of the present invention, it was discovered that a solder composition having a tin-silver alloy (e.g. 96.5% Sn-3.5% Ag) powder (the primary powder) mixed with additive powder of bismuth (Bi) (3-20% preferably 3-15% of total metal weight), silver (Ag) (3-11% of total metal weight), copper (Cu) (3-10% of total metal weight), or nickel (Ni) (3-10% of total metal weight), or combinations thereof, provides a solder which has improved fatigue and creep resistance and does not contain toxic ingredients. The phrase total metal weight is the weight percentage of a elemental or alloy metal in comparison to the weight of all of the metal components of the solder paste, but excluding the weight of the flux. A solder having 3-11% of total metal weight of bismuth has been found to be an especially effective additive powder for the tin-silver alloy.
It is also a desired object of the present invention to provide a solder exhibiting favorable thermal and electrical conductivities. Yet another object of the invention is to provide a solder composition that is affordable and in reasonable supply for large production use and that takes advantage of existing high volume production equipment and soldering processes. These and other advantages, features and objects of the present invention will become more apparent to those of ordinary skill in the art upon reference of the following description.